Engine exhaust systems utilize sensors to detect operating conditions and adjust engine air-fuel ratio. One type of sensor used is a switching type heated exhaust gas oxygen sensor (HEGO). The HEGO sensor provides a high gain between measured oxygen concentration and voltage output. That is, the output of the HEGO sensor is very close to being a step change in voltage at stoichiometry. Hence, the HEGO sensor can provide an accurate indication of the stoichiometric point, but provides air/fuel information over an extremely limited range. For HEGO sensors located upstream of the catalytic converter, the location of the characteristic step change may shift from stoichiometry as a result of system characteristics such as incomplete exhaust gas mixing.
Another type of sensor used is a universal exhaust gas oxygen sensor (UEGO). The substantially linear relationship between the sensor output voltage and exhaust gas oxygen concentration allows the sensor to operate across a wide range of air-fuel ratios, and therefore can provide advantageous information when operating away from stoichiometry. However, as recognized by the inventors herein, the UEGO sensor may not provide an indication of stoichiometry as precise as the HEGO sensor without the binary output of a HEGO to accurately locate the desired air-fuel ratio. For UEGO sensors located upstream of the catalytic converter, errors in perceived air-fuel ratio may occur as a result of system characteristics such as incomplete exhaust gas mixing. Furthermore, small variations in the output characteristic from sensor-to-sensor, or changes in the sensor characteristic with age or operating point, may cause a deterioration in the emissions performance of the system. Further, a typical UEGO calibration can have variance that is higher than desired for improved control results. Finally, the sensor's calibration may drift over time, degrading performance.
Several closed-loop air-fuel ratio control systems are known that utilize sensors upstream and downstream of a three-way catalytic converter (TWC) for controlling engine air-fuel ratio operation. Such systems may include various combinations of upstream and downstream sensors. In some approaches, upstream and downstream sensors are used to regulate the amount of oxygen stored in the TWC (see U.S. Pat. No. 6,502,389, for example). Regardless of the approach, a feedback signal on engine A/F is typically derived from the upstream sensor. The sensor downstream of the catalytic converter, considered to be unbiased, generates a signal used to correct the upstream sensor signal and maintain high efficiency catalyst operation. However, the inventors herein have recognized that a fundamental property of such systems is that if the aft sensor is miscalibrated, then it may not be possible to correct errors on the upstream sensor.
The inventors herein further have recognized that when an oxygen sensor is used in an exhaust gas system of an engine operating at a wide variety of conditions, the precise indication of stoichiometry given by the HEGO sensor provides advantageous results. In particular, conventional methods of correcting the setpoint of a pre-catalyst (UEGO or HEGO) sensor using a post-catalyst HEGO or UEGO sensor can require substantial calibration, and do not necessarily locate the setpoint of the upstream sensor at the highest possible conversion point of the catalyst.
To overcome these disadvantages, and harness the advantages of both types of sensors, the following approach can be utilized to calibrate a UEGO sensor against a HEGO sensor. In the absence of a chemical bias, for example in the case of a sensor located aft of a catalytic converter, this can yield a stoichiometric or other calibratible set-point.
Specifically, in one aspect, a method for controlling fuel injection into an engine having an exhaust system with an emission control device located therein is used. The method comprises:
reading information from a downstream sensor coupled in said emission control system downstream of said emission control device, said information including a substantially linear indication of exhaust air-fuel ratio across a range of air-fuel ratios from at least 12:1 to 18:1, said information also including a substantially non-linear indication of stoichiometry;
adjusting a setpoint for an upstream sensor based on said signal; and
adjusting fuel injection into the engine based on said adjusted setpoint and a signal from said upstream sensor.
In this way, it is possible to automatically establish a sensor setpoint (for example a setpoint corresponding to stoichiometry), even when using a sensor that provides a wide range air-fuel ratio sensing ability. Further, it is possible to determine a setpoint for an upstream sensor that accurately locates the point of maximum conversion efficiency with reduced calibration.
Also, since this example uses a method of extracting both switching and linear signals from a single sensor, it is possible to enable the identification of a UEGO sensor setpoint corresponding to stoichiometry without requiring a separate HEGO sensor.
An advantage of the above aspect is to obtain high catalytic converter efficiency despite sensor-to-sensor variability or changes in the sensor characteristics over time by adjusting the control setpoint during normal engine operation.